Hydrogenation is a critical process for improving electronic properties of polycrystalline silicon (referred to herein as "poly-Si") for thin-film transistors. Proper hydrogenation has been shown to passivate traps, reduce device leakage current, and improve drive current by increasing carrier mobility. The degree of poly-Si hydrogenation is often inferred from time consuming analytical measurements or from end-of-line transistor parameter characteristics. It has also been found that poly-Si hydrogenation can be monitored in a quick, non-contact manner by surface photovoltage (SPV) measurement, which provides a convenient real-time feedback for process control. This SPV approach is relevant to the effect of the ultrasound treatment described below.
The preferred embodiment of the present invention provides enhanced hydrogenation in poly-Si thin films by means of ultrasound treatment (UST) of such thin films. Ultrasound waves propagating through a semiconductor produce lasting changes in recombination properties related to point and extended defects. It has been found that UST applied to p-type Cz-Si and solar-grade polycrystalline Si caused significant changes in the dissociation-association kinetics of metal acceptor pairs. These results suggest that coupling of ultrasound vibrations to semiconductor materials will vary properties of point and extended defects not only in compound semiconductors, but also in silicon. Stimulated by UST, processes of point defect releasing from traps as well as point defect gettering by sinks can be of particular importance for polycrystalline silicon thin films. This is explained by two reasons: (1) existence of extended lattice defects (grain boundaries and dislocations), which effectively absorb the acoustic waves; and (2) existence of mobile point defects, e.g. hydrogen, strongly interacting with extended defects.
The present invention is particularly useful in providing enhanced hydrogenation in polycrystalline silicon (poly-Si) thin films on substrates such as glass substrates, for use in thin-film transistor ("TFT") applications. Specifically, in TFT manufacturing, plasma hydrogenation is normally one of the final technological steps. According to the present invention, UST processing is designed to enhance plasma produced hydrogenation of TFTs.
One aspect of the present invention relates to the application of UST to hydrogenated poly-Si thin films, to enhance the electronic properties of such poly-Si thin films.
Another aspect of the present invention relates to the application of UST to hydrogenated poly-Si thin film transistors to improve the electronic properties of such transistors.
Still another aspect of the present invention is the mechanism by which hydrogenation and application of UST to a poly-Si film improves its electronic properties. Specifically, hydrogenation and application of UST adds hydrogen to the poly-Si film and promotes passivation of dangling Si bonds in the film, to enhance the electronic properties of the poly-Si film.
The present invention will become further apparent from the following detailed description and the accompanying drawings.